Landing gear implementation is making ever-increasing use of electric actuators. These include actuators for operating undercarriages, wheel-bay hatches, and hook units that are all electrically actuated.
In general, hook units are fitted with two types of actuator, a main actuator and an emergency actuator for use in the event of the main actuator failing. It is important to be able to release the undercarriage retained by the locking hook, even in the event of the main actuator failing.
In configurations in which the emergency actuation is provided by an electromechanical actuator, it is appropriate to ensure that the emergency actuator can be powered. However, circumstances may arise in which the main electrical power supply networks of the aircraft are not capable of powering emergency actuators (e.g. a total failure of the engines of the aircraft). It is then appropriate to ensure that the emergency actuators can be powered, even under such circumstances.